The act of masturbating into an inside-out sandwich bag, often times using spit as lubrication.
It is critical that the sandwich bag is inside-out in order to prevent the process known as "seam chafing."
This act is generally performed simultaneously with light anal exploration, such as a finger in the anus.

Guy 1: "So I walked in on my roommate the other night Cooping in the living room."

Guy 2: "In the living room?"

Guy 1: "Yes, right in the middle of the living room. I'm scarred for life."